The evolutionary story of birch, told through 80 genomes

“Birch is one of the major trees for forest products in the Northern Hemisphere. Others, like spruce, pine and poplar, all have genome sequences, but birch did not — until now.”

Victor Albert, professor

Department of Biological Sciences

Forests of silver birch stretch across Europe, and they are a
wonder to behold: stands of slender, white-barked trees sheltering
vast swathes of earth.

But these woodlands also have value beyond their beauty: They
are an economic asset, generating raw material for papermaking,
construction, furniture-building and more.

A new study illuminates the evolutionary history of birch, a
tree that has not been studied much by scientists, despite its
commercial value.

“Birch is one of the major trees for forest products in
the Northern Hemisphere. Others, like spruce, pine and poplar, all
have genome sequences, but birch did not — until now,”
says UB biologist Victor Albert, who co-led the Finnish-funded
project with Jaakko Kangasjärvi, Ykä Helariutta, Petri
Auvinen and Jarkko Salojärvi of the University of Helsinki in
Finland. Helariutta is also a professor at the University of
Cambridge.

“We sequenced about 80 individuals of one
species, Betula pendula, the silver birch,” says
Kangasjärvi. “We sampled populations of this species
throughout its range, so up and down Finland, down to Germany, over
to Norway and Ireland, and all the way up to Siberia.”

By analyzing the 80 genomes sequenced, the team was able to
identify genetic mutations that may be of interest to industry,
including mutations that may affect how well birch trees grow and
respond to light at different latitudes and longitudes, and under
different environmental conditions.

The research could be a starting point for breeding trees that
better meet the needs of various industries.

“What makes a birch tree hardy in different environments?
A tree in Finland may die if you plant it in Siberia because plants
have local adaptations — specific genetic mutations —
that help them survive where they are found,” Helariutta
says. “An understanding of these natural adaptations can
facilitate genetic engineering and artificial selection.
That’s why our research could be very useful for forest
biotechnology.”

In the study, published May 8 in Nature Genetics, the
researchers identified genetic mutations of interest by hunting for
distinctive stretches of DNA within the genomes of individual birch
trees.

Like people, plants inherit two copies of every gene — one
from each parent — and these two copies are slightly
different from each other. However, in some spots, an organism may
have long strips of identical DNA in both copies of a gene. Such
stretches of DNA point to genetic regions that are critical to a
species’ survival and development, as these regions are the
product of “selective sweeps” in which all or most
organisms in a geographic location come to depend on a certain
genetic trait.

When the scientists analyzed the genomes of 80 birch trees from
across Europe, they discovered a rich array of selective sweeps in
genes that influence important qualities such as tree growth and
wood production.

Moreover, the team found that some selective sweeps appeared to
be associated with various environmental conditions. Two genes that
help control how birch trees respond to light — PHYC and
FRS10 — had notable genetic mutations correlating with
latitude, longitude and temperature, while the mutations in PHYC
were also related to precipitation trends. Similar associations
were also identified for two genes tied to wood production —
KAK and MED5A. Mutations in these genes were correlated with
latitude, longitude and temperature.

“The selective sweeps we identified may be the basis for
local adaptation for different populations of birch,”
Salojärvi says. “Trees in Siberia are under different
selective pressure from trees in Finland, so genes are being
tweaked in different ways in these two places to allow these plants
to better adjust to their environment.”

“The research points to genetic mutations that could be of
interest for genetic manipulation for forest products,”
Auvinen says.